β-Casein was quantified in milk and cheese, using an optical immunosensor, based on surface plasmon resonance (SPR) measurement. The assay consists of a two-step sandwich strategy, with two anti-β-casein antibodies directed against each extremity of the casein. This strategy permits only native β-casein to be quantified and not its degradation products. The calibration curve was obtained with a reference milk powder of known β-casein concentration. The analysis time per sample was less than 10 minutes. The antibody-coated surface could be used for more than 250 determinations. The detection limit was established at 85 ng ‚ mL-1 and the intra-and inter-assay variation coefficients were 2.6 and 6.2% respectively. The method was applied to raw milk to quantify intact β-casein, with no pretreatment of the sample. A second application was realized with cheese, to follow the proteolysis of β-casein during ripening.